1. Field of the Invention
The present invention is related to a method for detecting a condition of a semiconductor wafer, and more particularly, to a method for detecting the condition by using scatterometry critical dimension metrology.
2. Description of the Prior Art
With the advancements of wafer manufacturing processing, advanced lithography makes it possible to mass manufacture semiconductor elements that have line widths of less than 100 nano-meters. However, while the scale of semiconductor is reduced, the critical dimensions (CD) of channel length, junction depth, and gate thickness of a field effect transistor (FET), for example, are reduced, as well. These critical dimensions should be accurately controlled because a little change in the critical dimensions may cause a lot of change in the characteristics of the semiconductor element. Therefore, the measurements of critical dimensions with advanced lithography will become more and more important in the future. Rapid and repeatable measurements will be used to detect the characteristics of semiconductor products on a production line. The accuracy of the measurements influences the yield and the reliability of the semiconductor products.
Some favorable metrologies for semiconductor process include advanced bright-field microscopy, scanning electron microscopy (SEM), scatterometry, and atomic force microscopy (AFM). Among them, the scatterometry metrology provides much progress in critical dimension measurement and profile analysis. Scatterometry can provide more information than other metrologies, such as pitch, sidewall angles, thickness, and the thickness of the underlying thin film. Therefore, the scatterometry CD metrology has been widely used in the photolithography process to provide the critical dimension measurement and profile analysis. The advantages of using scatterometry in the photolithography process lie in its non-destructive measurement, the ability to measure the critical dimension as small as 40 nm, and installation without expensive equipment.
The major principle of scatterometry is that the intensity of incident light of the periodic gratings varies with the incident angle or with the wavelength of the incident light and the relationship between reflectance and angle/wavelength can be recorded as different signatures. Scatterometry can be divided into forward metrology and reverse metrology. Forward metrology is to measure the spectra of the laser beam reflected by the periodic grating. Reverse metrology is to compare the measured spectra with the theoretical spectra model to provide the structure information of the grating. The reverse metrology can be further divided into library matching method and direct regression method. The library matching method compares the measured data with the data in a library to find the closest grating structure. The direct regression method compares the measured data with a theoretical model, and modifies the parameter to gradually reduce the difference between measured and theoretical data.
In the prior art, only the impact on the critical dimensions from limited process parameters is analyzed, such as the relationship between the exposure energy and the critical dimension. Most of the other process parameters, such as post-bake temperature and illumination conditions, are seldom discussed and it is difficult to know their impact on the critical dimension.